I couldn't believe it when I read your post. I had just finished getting my husband to sit up straight again! Only this week he has started leaning to the left whenever he sits for any length of time. I've tried propping him with cushions etc but he still manages to lean.
I'll look forward to reading the experiences of others.
My mum has in the last 6 months started leaning to the right when seated. She's left-handed, if that is of any relevance? She is now almost unable to walk and uses a wheelchair most of the time. Sounds as if our relatives are at a similar stage physically. Wish I knew what caused the leaning, but I don't think it can be corrected. It seems to happen to a lot of people. Perhaps it is the start of muscular contractures?